The present invention relates to an apparatus and method for feedwater heating in an industrial steam turbine.
In known condensing reheat steam turbines, the working steam discharges the residual heat in a condenser to the surroundings after expansion of the steam in the last turbine stage. This discharge produces a cold condensate which is reheated by a multistage heating unit using steam extracted from the turbine.
In industrial plants, steam which is used for heating purposes must have a varying pressure level depending on the application. Before the steam is utilized for industrialized heating purposes, the steam produces mechanical energy in the turbine or, as the case may be, electrical energy in a generator coupled to the turbine. With a combined energy utilization of the steam, i.e., heat-power coupling, it is possible to achieve a relatively high degree of utilization of primary energy of the steam.
These known industrial plants are disadvantageous in that condenser components for small plants operate quite uneconomically.
For this reason, in backpressure or backpressure pass-off turbines, the steam is expanded in the turbine to the level of the lowest process steam pressure.
In these plants, it has been found to be disadvantageous to have the condensate flowing back from the steam users (for example in the paper industry from the cookers and paper machinery) into the heater because the condensate is, in most cases, heavily contaminated and also must be further cooled. With a recirculation temperature of over 70.degree. C., for example, the condensate must be further cooled down before running through a purification plant. For this reason, a purification step is waived in most industrial plants and the contaminated condensate is simply discharged. The quantity of condensate which is lost thereby must be replaced by purified fresh or makeup water. Since this fresh or makeup water is cold, it must be heated up to a temperature suitable for the steam boiler. Particularly in the case of backpressure or backpressure pass-off turbines, the steam at a pressure level of the process steam network is used for the heating step. However, if the pressure of the lowest process steam network is relatively high, large losses can occur as a result of this heating step.
It is an object of the present invention to provide a device for feedwater heating in industrial steam turbines in which one portion of the steam is expanded to a lower pressure level than that of the lowest process steam network. The further expanded steam is to be used for heating a purified condensate and feedwater.
The above-mentioned object and others are achieved according to the present invention by providing at least one further turbine stage downstream from the last turbine stage for expanding one portion of the process steam. Additionally, at least one additional feedheater with lower pressure is provided.
The apparatus according to the present invention provides a double pass-off backpressure turbine for supplying two process steam networks with steam. The present invention provides an advantage over known plants of this type which require condenser components for efficient feedwater heating. The present invention provides efficient feedwater heating without entailing the costs associated with a large condenser component or an entire condenser plant. Further, with the present invention, it is possible to achieve an increased steam turbine capacity with a given process steam quantity. Also, the temperature differences in the feedheating process are decreased resulting in diminished energy losses, i.e., the loss of the utilizable energy from the steam is diminished.
According to a further feature of the present invention, a flow control element is installed in a live steam line upstream from the steam intake into the steam turbine in order to maintain the lowest feedheater pressure constant.
By utilizing the flow control element, the feedheating works independently of the process steam quantities taken from the turbine. Therefore, extraction steam heaters connected downstream automatically reach almost constant pressure without control.